Recent investigations have shown that organic-inorganic hybrid silica membranes based on the precursors 1,2-bis(triethoxysilyl)methane (BTESM), or 1,2-bis(triethoxysilyl)ethane (BTESE) in the optional presence of methyltriethoxysilane (MTES) are suitable for the separation of water from several organic solvents, including n-butanol (Castricum et al. 2008-a, Sah et al., WO 2007/081212, Kreiter et al. WO 2010/008283). The long-term stability of these membranes was unprecedented. Membrane life-times up to 1000 days were demonstrated at an operating temperature of 150° C. (Van Veen et al. 2011). It was known that inorganic silica and methylated silica membranes do not survive at these temperatures (Campaniello et al. 2004). The performance of organic-inorganic hybrid silica is not affected by traces of nitric acid in a butanol/water mixture (Castricum et al. 2008-b) and acetic acid in an ethanol/water mixture (Kreiter et al, 2009; WO2010/008283).
Several authors have reported the separation of acid-water mixtures using pervaporation. In these reports water removal from acetic acid using pervaporation membranes is a key example process (Aminabhavi et al. 2003; U.S. Pat. No. 7,045,062).
Applications with a high organic acid concentration include esterification reactions in which e.g. acetic acid reacts with ethanol. In situ removal of the by-product water leads to increased conversion and higher product quality (Iglesia et al. 2007).
Amorphous silica membranes are selective for water separation from acetic acid at different mole fractions (Ishida et al. 2006). Although the membrane performance was demonstrated at individual compositions, the long-term performance and stability of these inorganic silica membranes were not demonstrated. Moreover, it is well-known that amorphous silica-based materials change structure in the presence of water and acid. This is due to proton-catalysed hydrolysis and condensation processes, the rates of which are related to, amongst other factors, the acid concentration. For pervaporation membranes based on silica and methylated silica, membrane performance thus declines within several days of operation in hydro-organic mixtures, even at temperatures of 95° C. and lower (Campaniello et al. 2004). This decline in performance can be recognised by a decrease in selectivity and separation factor that is often accompanied or preceded by an increase in water and organic solvent flux.
Zeolite membranes such as Silicalite-1 applied in the separation of water from acetic acid show low fluxes and limited separation factors, especially at lower water concentrations (Masuda et al. 2003). For example, the separation factor of this Silicalite-1 membrane (calculated according to equation 1) found for a mixture of 90/10 wt % acetic acid/water is around 10. This zeolite membrane is stable for 24 h of operation at 70-80° C. The stability over longer periods of time is unknown. Zeolite A in particular is highly unstable in the presence of acids, as was demonstrated by membrane reactor experiments in the esterification of acetic acid (Iglesia et al. 2007). The same authors describe mordenite membranes having an acetic acid/water separation factor that reaches a maximum of 98 during a total measuring time of 5 days at 80° C., starting from an equimolar mixture of acetic acid and ethanol. Related work demonstrates that higher concentrations of acetic acid (HAc) are detrimental for the separation factor. At a feed concentration of HAc/H2O 90/10 wt % the separation factor drops to about 50. Application temperatures beyond 90° C. were not reported. A further well known disadvantage of Zeolite A is the limited stability in hot liquid water, limiting the application of the membrane at high water contents in any mixture including that of water with an alcohol or an organic acid.
Another zeolite membrane is hydroxy sodalite. This membrane was stable at high water content, e.g. at HAc/H2O 20/80 mol %, for at least 15 days. However, when the acid concentration was increased to HAc/H2O 31/69 mol %, the membrane failed within 20 hours. (Khajavi et al, 2010).
The capability of separating water from acetic acid has been reported for several polymer membranes (Aminabhavi et al. 2003). Some examples are polyvinyl alcohol (PVA), PVA blends, and cross-linked PVA membranes. All of these PVA-based membranes are limited in application temperature to about 80° C. because of the glass transition temperature of the polymer at 85° C. In addition, acetic acid is a strong plasticizer for PVA, especially at higher acetic acid mole fractions (Aminabhavi et al. 2003). Alternative polymer materials suffer from the same glass transition limit in the region of 70-85° C. Polyimide membranes are suitable for higher temperature applications (Zhou et al. 2006-a, 2006-b). However, the separation factors obtained with a native polyimide membrane based on the polymer Matrimid are limited to 15-35. The separation factor can be increased to 95 by a heat treatment of the Matrimid membrane, but only at the expense of water flux, which is lowered by a factor of 3 to 1.65 kg/m2·h. In addition, Matrimid and polyimides in general are not sufficiently stable in the presence of water even in the absence of an acid (Kreiter et al. 2008).
Tsuru et al. 2012 disclose an organic-inorganic hybrid silica membrane produced by first coating an α-alumina support with titania, followed by coating with silica-zirconia and finally coating with BTESE. The resulting microporous membrane appears to have an intermediate titania and/or silica/zirconia layer in the order of 1 μm thick, which, according to the Knudsen diffusivity will be a microporous or mesoporous layer. After testing the membrane in dewatering acetic acid at a feed composition of HAc/H2O 90/10 wt % at 75° C. with good performance, the separation factor decreased while the water flux increased, showing an irreversible decline in membrane performance. This indicates that high temperature stability is low, although long term stability was reported at room temperature. Therefore these membranes do not appear to be suitable for dewatering of acids by pervaporation at high temperatures for periods of time relevant to the industry.
US 2002/0142172 discloses a dual-layer inorganic microporous membrane comprising a surfactant-templated intermediate layer. An intermediate mesoporous γ-alumina layer (pore size 5 nm) is always present between the macroporous substrate and the templated intermediate layer. The microporous top layer can be made from organic silanes such as bis-triethoxysilyl-ethane, but the ethane ligands embedded in the silica framework are removed by calcination at 280° C. for creating micropores. Thus the final membrane deliberately does not contain organic bridges in the silica. US 2002/0142172 does not suggest separation of water from acid or vice versa, and its membranes will be unsuitable for such separation. Tsai et al. 2000, which corresponds to US 2002/0142172, confirms the intentional removal of ethane bridges.
Thus, there is a need for hybrid inorganic-organic membranes which do not exhibit acid sensitivity even at high acid concentrations and high temperatures after prolonged periods of several weeks or months.